[CURA-5767]
The backend can't handle per mesh-group settings (as opposed to per mesh settings)
when sequence was in one-at-a-time mode. The problem is that the extruder train is
in between the per-meshgroup-settings (child-of) and per-mesh-settings (parent-of).
For any per-mesh-group settings, the possibility exists that they'll be overridden
by settings we always set (and that need to be set) per extruder. Changing this
would require the engine-architecture to change, and _all_ settings would need to
be thouroughly retested. As this was a too extensive change, it was decied just to
disable the per-meshgroup settings when printing one-at-a-time (too).
The issue was originally reported as:
'"Printing Temperature Initial layer" setting per mode does not work'